The present invention relates to a method and apparatus for controlling an array of light-emitting elements to create an intensity-graded image on a photosensitive medium.
Such arrays are employed, for example, in printers, copiers, and similar devices in which the photosensitive medium is a revolving photosensitive drum and the array is a linear array of light-emitting diodes (hereinafter, LEDs). The image formed on the drum is an invisible image consisting of electrical charges, which is typically developed by applying toner particles, then transferred to paper.
To print color images or halftone monochrome images, the light-emitting elements must produce an intensity-graded image. This is conventionally done by controlling the voltage at which the light-emitting elements are driven, thereby controlling the intensity with which they illuminate the photosensitive medium. A drawback of this method is that the intensity data are normally provided in digital form, from a computer, for example, so it is necessary to convert the digital data to a graded range of analog voltages. A separate digital-to-analog converter for this purpose must be provided for each light-emitting element. These digital-to-analog converters require complex circuitry, typically employing a precision resistor ladder that is difficult and hence costly to fabricate. Further, the digital-to-analog converters take up considerable space in an integrated circuit.